Dual stage membrane gas separators with variable conductance means for varying their throughput



7. 1968 P. M. LLEWELLYN 3,393,505

DUAL STAGE MEMBRANE GAS SEPARATORS WITH VARIABLE CONDUCTANCE MEANS FOR VARYING THEIR THROUGHPUT Filed March 27, 1957 FIG.| PRIOR ART DUAL STAGE HIGH MASS GQQ spacmoram SEPARATOR l l r4 RECORDER FROM GAS CHROMATOGRAPH 2 i l l5 l2 3 FIG.2 5 7 MASS 57 W SPECTROMETER HIGH "q "I H VACUUM 119W PUMP (Z ;/7/////// I8 Q 15 27 -2L z A 2 VACUUM RECORDER VENT PUMP IN VEN TOR.

PETER M. EWELLYN BY TIMHMIN.) ATTORNEY FIG. 3

GAS CHROMATOGRAPH o OUTPUT United States Patent 3,398,505 DUAL STAGE MEMBRANE GAS SEPARATORS WITH VARIABLE CONDUCTANCE MEANS FOR VARYING THEIR THROUGHPUT Peter M. Llewellyn, Menlo Park, Calif., assignor to Varian Associates, Palo Alto, Calif., a corporation of California Filed Mar. 27, 1967, Ser. No. 626,194 6 Claims. (Cl. 55--16) ABSTRACT OF THE DISCLOSURE Dual stage membrane type gas separators are disclosed along with gas analyzing systems employing same. In such separator, the gas pressure on the output side of each successive membrane is lower than that on its input side. It is disclosed that a variable conductance device is connected in the exhaust tubulation through which the space between two successive membranes is evacuated. By controlling the conductance of the variable conductance device the gas pressure in the space between the membranes is controlled, thereby controlling the percentage throughput of the two stage separator. The two stage gas separator with adjustable throughput is connected in line between a gas chromatograph and a mass spectrometer type gas analyzer. The separator enriches the gas sample by discriminating against the carrier gases. In addition, the variable conductance device permits adjusting the sample throughput to the mass spectrometer over a range from 80% to 2%. Thus, small and large output peaks of the gas chromatograph may be analyzed by the mass spectrometer without overloading the mass spectrometer by preselecting the throughput of the separator via the variable conductance device.

DESCRIPTION OF THE PRIOR ART Heretofore dual stage membrane gas separators have been employed for separating the carrier gases of a gas chromatograph from the output peaks to be analyzed by a mass spectrometer. Such a prior art system is described and claimed in copending U.S. patent application 563,235 filed July 6, 1966, as a continuation-in-part of U.S. patent application 511,794 filed Dec. 6, 1965, and both assigned to the same assignee as the present invention.

In these prior systems, the gas conductance of the exhaust tnbulation through which the space between successive memberane was evacuated was selected at a fixed value. In such a case, the membrane separator had a fixed percentage throughout to the mass spectrometer. The problem with this arrangement at this the output of the gas chromatograph is characterized by a time sequence of gas bursts of the different constituents of the sample under analysis as charged to the gas chromatograph. These gas bursts or peaks are spaced in time by as little as a few seconds to as much as several minutes. Also the size of the gas burst or peak varies considerably. The prior art separator passed a fixed percentage of the gas peaks to the mass spectrometer. Thus, the large peaks could overload the spectrometer with gas and prevent identification of the smaller peaks. If the selected percentage throughput was fixed at a sufliciently small percentage to avoid overloading the mass spectrometer, the small peaks could not be identified.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved dual stage gas separator and systems using same. u

One feature of the present invention the plOVlSlOIl,

3,398,505 Patented Aug. 27, 1968 in a dual stage membrane gas separator, of means for variably controlling the gas conductance from the space between the two membrane separator stages to a device pulling gas therefrom, whereby the gas pressure between the membrane separators is readily varied to control the throughput percentage of the gas separator.

Another feature of the present invention is the same as as the preceding feature wherein the gas separator is included in-line between a gas chromatograph and a mass spectrometer, whereby the throughput of the gas separator may be readily controlled to prevent overloading of the mass spectrometer while monitoring relatively small gas peaks in sequence after relatively large peaks.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram of a prior art system employing a gas chromatograph, dual stage membrane gas separator, and a mass spectrometer,

FIG. 2 is a schematic diagram, partly in block diagram form, of a gas analysis system employing features of the present invention, and

FIG. 3 is a chromatogram (graphical representation of an output of a gas chromatograph).

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a prior art gas analyzing system. The system includes a gas chromatograph 1 into which is charged a sample to be analyzed. The output gas flow of the gas chromatograph 1 comprises a time sequence of gas bursts or peaks carried along in a stream of inert carrier gas. The difierent peaks represent different gaseous constituents of the sample under analysis.

The output gas stream is fed to a dual stage membrane type gas separator 2. The separator 2 is more fully described below and in the aforecited patent application 563,235. The gas separator 2 nearly eliminates the carrier gas thereby greatly enriching the percentage of the sample constituents in the gas flow output of the separator 2. The output of the separator 2 is fed to a gas analyzer such as a mass spectrometer 3. The spectrometer may be any of several types such as, for example,

a magnetic sector type or a cycloidal type. The output.

of the mass spectrometer is recorded by a recorder 4 to identify the various constituents of the sample under analyses. A high vacuum pump pull-s the gaseous constit-' uents through the mass spectrometer.

The problem with the prior art system of FIG. 1 is that the output of the gas chromatograph 1 has peaks of greatly differing size as shown in FIG. 3. In the past, the gas separator 2 has been set to pass a certain fixed percentage of the sample constitutents through to the mass spectrometer. This means that the sample flow rates into the mass spectrometer vary widely in accordance with the relative amplitudes of the peaks of FIG. 3. Thus, if the separator is set to pass a fixed percentage of the sample which is sufiicient for the mass spectrometer to identify the small peaks, the large peaks which pass into the spectrometer flood the spectrometer with sample material and prevent detection of small peaks occurring within the flooded time of the spectrometer which can be several minutes. On the other hand, if the fixed percentage flow rate of sample to the mass spectrometer is set sulficiently low to prevent flooding by the large peaks, the small peaks provide too little sample material to the spectrometer to be identified.

---Referring new to FIG." 2, there is' shown thegas handling and separating system of the present invention. The apparatus is essentially identical to that of FIG. 1 except that .a variable throttle valve 11 is provided for varying the percentage-of gas throughput of the gas separator 2. More specifically, a gas passageway 12, as of l" in inside diameter, has an inlet pipe 13 at one end which is connected tothe output gas flow of the gas chromatograph 1. A vent pipe 14 is connected to the conduit 12 opposite the inlet pipe 13. A cap 15 closes off the input end of the gas passageway 12. Gas fiows into the passageway 12 via inlet pipe 13 at atmospheric pressure and is exhausted via vent pipe 14 at atmospheric pressure.

A pair of gas separating membranes 16 and 17 are sealed across the gas passageway 12 in spaced relation taken-in a direction along the gas passageway 12. The membranes 16 and 17 each comprise, for example, a 1 mil, thick polysiloxane polymer membrane supported on a perforateddisk asof glass. Preferably, the membranes 16 and 17 are constructed from materials selected from the group consisting of polymers and stationary liquid phases. Stationary liquid phases are those liquid materialsemployed in chromatographic columns to partition materials to be separated. Comprehensive lists of such materials can be found in numerous publications, one being Gas Chromatography by Ernst Bayer, publishedby Elsevier Publishing Company, New York, 1961, Tables 2, 13 and 14.

Polymers and stationary liquid phases are characterized by generally being free of holes. Hence, gaseous state material can pass through such material only by diffusion. However, in order to diffuse through the membrane, the gaseous state material must first be captured by the membrane either by entering into solution therewith or adhering thereto. Although most gases can be captured by such membrane materials, the permanent gases generally will not be captured, especially at elevated temperatures, i.e., substantially above zero degrees centigrade.

Thisproperty of the membrane materials is employed to facilitate analyzing selected gaseous state materials. In operation a partial pressure difference for the gas to be passed is established between opposing surfaces of the membrane. The material which is to be analyzed is communicated to the surface of the membrane at the higher partial pressure. Gaseous state materials, except the permanent gases, will readily enter into solution with the membrane material and diffuse therethrough. The gaseous state materials which diffuse through the membrane may then .be directed to any suitable gas analyzer for analysis. It is important to note that the ratio of the gases of a mixture passed by the membrane is independent of pressure and gas flow fluctuations.

In those instances where stationary liquid phases form the membranes 16 and 17, a suitable reservoir supporting structure must be provided. For example, a reservoir constructed from a polymer or a fine screen mesh capable of supporting the liquid by surface tension would be suitable. However, other materials will work if relative to gaseous state materials they are free of holes, and if the permanent gases will not enter into solution with the material. As utilized herein, entering into solution is defined as a process of condensation and then mixing of the gaseous state material in the surface layers of membranes 16 and 17. (See Physics and Chemistry of the Organic Solid State, edited by David Fox, Mortimer M. Labes and Arnold Weissberger, published by Interscience'Publishers, NewYork, 1965, vol. 2, p. 517.)

The membranes 16 and 17 operate as follows: The carrier gas which is any one of a number of permanent gases such as N He, H Ar, etc. is not soluble-in the elastomer film membrane. On the other hand, the organic vapor constituents of the sample, which are carried in the carrier gas, are soluble in the elastomer film membrane. These organic vapors go into solution with the elastomer and diffuse through the membrane under a partial pressure differential thereacross and come -out'-of solution on the downstream side of the membrane. Since the organic vapor passes through the membrane and the carrier gas is largely excluded a substantial enrichment of the organic vapors in the carrier gas stream is obtained.

The permeability of the membrane for a given gas can be calculated from the following expression: a

where S is the solubility of the gas in the membrane, D is its diffusion rate through the membrane, A is area of the membrane, p p is the partial pressure difference across the membrane for the gas of interest, at is thickness of the membrane, and t is time. For the aforedescribed membrane of an area of about 5 cm. the conductance for various gases at 30 C. is as follows:

Permanent gases:

cc./sec.

Ar 001 Organic vapor (n-octane):

A vacuum pump 18 is connected into the region 19 of the conduit 12 between the two membranes 16 and 17 via an exhaust tubulation 21. The throttle valve 11 is placed in the tubulation 21 to form a variable conductance in the gas passageway between the pump 18 and the chamber 19. The vacuum pump 18 may be of the mechanical type to provide a base pressure, as of 10- torr. Varying the throttle valve setting varies the conductance of the pas sageway and, thus, the pressure in the chamber 19.

The throttle valve comprises a stainless steel slug 22 which has a permanent magnet 23 afiixed at one end. The slug 22 is slidable in an arm 25 of a T fitting 24. The outside of the T fitting arm 25 is threaded to receive the inside threads on a cylindrical magnet 26. The magnet 26 is formed by two oppositely polarized semicylindrical magnets cemented together in slightly spaced relation to form a hollow cylindrical member which is then internally threaded. Turning the magnet 26 on the arm 25 causes the magnet 26 to move axially of the arm 25 and,

then, to cause the internal slug 22 to move toward or away from the port, at 27, leading into the other arm of the T, thereby varying the conductance of the tubulation 21.

The gas throughput from the second membrane 17 is fed to the mass spectrometer 3 for analysis. The high vacuum pump 5 maintains the gas pressure on thedownstream side of the second membrane 17 at a pressure of about 10- torr or' less. In a typical example, the gas conductance to the mass spectrometer 3 is about 1 liter/sec. and the carrier gas flow for He is about 10- torr-liters] sec. The organic vapor enrichment of the carrier gas flow is about 10 to 10 By varying the gas conductance of the gas passageway 21 by means of the throttle valve 11, the percentage of organic vapor passed by the gas separator to the mass spectrometer 3 can be varied over a range from to' 2%. This range of variation is extremely useful in operation of the gas analyzing system as it permits the operator to examine and identify chromatograph peaks without flooding the spectrometer 3. More specifically, the operator can observe the ohromatogram' output recording of the gas chromatograph 1. When a large peak appears in the output of the chromatograph which'is likely to flood the spectrometer 3, the operator can change the setting of the throttle valve 11 to reduce the flow of the sample organic vapors into the spectrometer 3. Likewise, when a small peak occurs in the output of the gas chromatograph which it is desired to identify, the operator changes the setting of the throttle valve to pass more of the sample into the mass spectrometer 3.

In a typical gas analyzing system of FIG. 1 the input charge to the gas chromatograph 1 is about 1 mg. This charge is spread out over a period of about 200 seconds in the output of the chromatograph 1 such that the output is about 5 g/ sec. The separator 2 reduces the sample charge to the mass spectrometer 3 to about 0.1 g/sec. The typical operating range for sample charge to the spectrometer is from 0.001 g/sec. to 0.1 ,ug./sec. The throttle valve 11 permits economical use of sample material and, in addition, increases the operating range of the gas analyzing system.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. In a gas separator, means forming a gas passageway, means forming first and second gas separating membranes sealed across said passageway in spaced relation taken along a direction of gas flow in said passageway, means for partially evacuating the region of said passageway between said spaced first and second membranes, said partial evacuating means including a second gas passageway in gas communication with the region between said spaced first and second membranes, the improvement comprising, throttle valve means for variably controlling the gas conductance of said second gas passageway, whereby the percentage gas throughput of the gas separator is variably controlled in response to changes in the gas conductance of said second gas passageway.

2. The apparatus of claim 1 including, means forming a vacuum pump connected to said second gas passageway for pulling gas from the region between said membranes through said variable conductance means and said gas passageway.

3. The apparatus of claim 1 including a gas chromatograph having its output gas peaks fed into said first gas passageway upstream of said first and second gas separating membranes, and a gas analyzer means connected in gas communication with said first gas passageway downstream or' said first and second gas separating membranes.

4. The method for analyzing the output gas flow of a gas chromatograph comprising the steps of, passing an output gas flow of the gas chromatograph having gas peaks of different sizes through a gas passageway having first and second gas separating membranes spaced apart along the gas passageway, partially evacuating the region of the gas passageway between the first and second membranes, passing at least a portion of the gas which has passed through the first and second separating membranes to a gas analyzer for analysis, and Varying the gas throughput of the second gas separating membrane in variable accordance with the size of certain ones of the gas peaks of the gas chromatograph to prevent excessive gas fiow to the gas analyzer.

5. The method of claim 4 wherein the step of varying the gas throughput of the second gas separating membrane comprises varying the gas pressure in the region of the gas passageway between the first and second gas separating membranes.

6. The method of claim 4 including the step of obtaining a mass spectrum of the gas fed to the gas analyzer.

No references cited.

REUBEN FRIEDMAN, Primary Examiner.

C. N. HART, Assistant Examiner. 

